Biosensing with holographic smart chips

快速检测化学和生物分子的能力对于从生化分析、医学诊断、药物筛选和国防等各种应用至关重要。大多数目前的生物传感器，例如DNA芯片或酶联免疫吸附分析，都依赖于用荧光或放射性标签标记被研究的样本，这既昂贵又耗时，而且可能会扰乱正在研究的蛋白质的三维结构。虽然替代的无标记技术，如石英晶体微天平和表面等离子体共振，通常受到其质量分辨率的限制，但最近的研究表明，当生物分子的皮摩尔作用于微制造硅梁的一侧时，平面内表面应力会导致悬臂弯曲。这种新颖的纳米机械驱动机制具有重要的优势，因为悬臂采用标准的低成本硅技术进行微制造，凭借其可实现的尺寸，对检测小分子化学和生物相互作用非常敏感，可以检测DNA和蛋白质的毫微米分子。在伦敦纳米技术中心，我们与Veeco Instruments Inc.合作开发了英国第一个多阵列悬臂生物传感器系统。然而，这项技术依赖于使用激光束测量固定悬臂自由端的挠度。因此，检测多个生物分子的能力受限于可微制造的固定端悬臂梁的数量。此外，物理测量仪器无法脱离生化环境，使日常临床使用具有挑战性。此外，在所有基于固定阵列的组合方法中，放大是通过增加阵列中元素的数量和密度来获得的，化学交叉污染和物理串扰是重要的障碍。我们的推测性工程方案承诺通过将精细的测量工具与更粗糙的生物力学活性元素悬臂完全分开来解决所有这些问题。我们寻求实现的根本出发点是将悬臂从传统方案中锚定它们的衬底上解开，并简单地测量它们在溶液中是自由对象时的弯曲。其基本想法是使用先进的光学技术同时测量芯片在流经狭窄通道时翻滚时的几个点的表面法线，并由此重建弯曲。我们的方案将能够在单个流体样本中搜索许多不同的寡核苷酸和蛋白质，因为不同的生物受体可以被拴在不同的芯片上，并且芯片本身可以被独特的全息标签标记。在临床应用中，我们设想将数万个这样的芯片与生物溶液混合，然后通过带有车载集成光学和相关数字处理的微流控读取器(本质上是一个微型超市条形码扫描仪)泵送产生的浆液来执行形状确定。总而言之，使用全息智能芯片快速检测生物分子的能力为高通量蛋白质组学、生物传感和医疗诊断提供了一种全新的方法。

The ability to rapidly detect chemical and biological molecules is essential for diverse applications, ranging from biochemical analysis, medical diagnostics, pharmaceutical screening and defence. Most current biosensors, for example DNA-chips or enzyme-linked immunosorbent assays, rely on labelling the sample under investigation with a fluorescent or radioactive tag, which is expensive, time consuming and can potentially perturb the three-dimensional structure of the protein under investigation. While alternative label-free technologies, such as quartz crystal microbalance and surface plasmon resonance, are typically limited by their mass resolution, it has recently been shown that when picomoles of biological molecules react on one side of a microfabricated silicon beam, in-plane surface stresses cause the cantilever to bend. The novel nanomechanical actuation mechanism has important advantages, because cantilevers are microfabricated by standard low-cost silicon technology and, by virtue of the size achievable, are extremely sensitive to detect small molecule chemical and biological interactions, detecting femtomoles of DNA and proteins.In the London Centre for Nanotechnology we have co-developed the first multiple array cantilever biosensor system in the UK in collaboration with Veeco Instruments Inc. However this technology relies on measuring the deflection of the free-end of a fixed cantilever, using a laser-beam. The ability to detect multiple biomolecules is therefore limited to the number of fixed-end cantilevers that can be microfabricated. In addition, the physical measurement apparatus cannot be separated from the biochemical environment, making everyday clinical use challenging. Also, as in all fixed array-based combinatorial methods where scale-up is derived from increasing the number and density of elements in the array, chemical crosscontamination and physical cross-talk represent significant hurdles. Our speculative engineering proposal promises to resolve all of these problems by separating the delicate measurement tool entirely from the much cruder biomechanically active elements, the cantilevers. The radical departure that we are seeking to implement is to untether the cantilevers from the substrate which anchors them in the traditional scheme, and simply to measure their bending when they are free objects in solution. The underlying idea is to use advanced optics to measure the surface normals at several points simultaneously as the chips tumble in a flow moving through a narrow channel, and from that, to reconstruct the bending. Our scheme will be able to search for many different oligonucleotides and proteins in a single fluid specimen because different biological receptors can be tethered to different chips, and the chips themselves can be labelled with unique holographic tags. In clinical application, we envision the mixture of tens of thousands of such chips with the biological solution, followed by pumping of the resultant slurry through a microfluidic reader (essentially a miniaturised supermarket barcode scanner) with on-board integrated optics and associated digital processing to perform the shape determination. To conclude, the ability to rapidly detect biomolecules using holographic smart chips forges a radically new approach to high-throuput proteomics, biosensing and medical diagnostics.